The present invention relates to a method of weighing a load on a balance equipped with load-receiving devices for loads of different shapes and sizes, wherein the load is set down in at least one of three locations, one of which is a stationary holding location, another location is a load-handling device, and a third location is a load-receiver portion of the balance. In particular, the balance is a mass-comparator balance, and the load consists of weights or mass standards of different shapes and sizes. The load-receiver portion supports the weight of the load and channels the weight force to a force-measuring transducer.
When a balance is used in a laboratory to perform a series of measurements, the load is often brought from a stationary holding location to the balance by means of a load-handling device (robot). The purpose of using a load-handling device is, on the one hand, to gain efficiency through automation and, on the other hand, to avoid the manual handling of loads which, in the specific case of calibration laboratories, will include small precision weights such as wire weights, sheet-metal weights or button-handle weights. In the case of a comparative measurement on a so-called mass-comparator balance, the load can be constituted by a plurality of weights that have to be picked up from the stationary holding location and brought to the balance. Balances of this kind are used for a process of weight comparison and are therefore equipped with a plurality of devices for receiving the weighing load. The latter are constituted in general by weighing pans arranged in a horizontal plane and acting on a common force transducer. Using a balance of this kind requires particular diligence, because a shift in the center of gravity can lead to errors in the result of the measurement. However, having to work with special diligence reduces the speed at which the process can be performed.
As an additional difficulty, the individual load receivers are radially offset from a vertical axis through the center point of load application of the force transducer, resulting in a so-called eccentric loading error, which is caused by a shift of the combined center of gravity when different weights are placed in different locations. Some balances have load-centering weighing pans equipped with a device that seeks to bring the center of gravity in line with the central axis, e.g., through an arrangement where the pan floats on ball bearings in a shallow spherical shell. However, this solution is not always satisfactory.
It is therefore the object of the present invention, to speed up the process of operating a balance, specifically a comparator balance, while at the same time reducing the potential for errors of the aforementioned kind. In particular, the present invention aims to avoid eccentric loading errors in balances where the weight is applied through a plurality of load-receiver devices.
In accordance with the present invention, the foregoing objective can be met by using a method of weighing a load on a balance equipped with load-receiving devices for loads of different shapes and sizes, wherein the load is set down in at least one of three locations, one of which is a stationary holding location, another location is a load-handling device, and a third location is a load-receiver portion of the balance. In particular, the balance is a mass-comparator balance, and the load consists of weights or mass standards of different shapes and sizes. The load-receiver portion supports the weight of the load and channels the weight force to a force-measuring transducer. The distinguishing feature of the invention is that the at least one location where the load is set down has at least two load-receiving positions arranged vertically above each other.
By using a vertical arrangement of load positions, it is possible to handle the loads of the vertically stacked positions together. Also, the combined center of gravity of the vertically stacked loads will essentially not shift around horizontally, regardless of how many of the vertically stacked loads are handled simultaneously.
An even greater advantage is gained from the inventive concept, if the load is set down in at least two of the locations one of which is a stationary holding location, another location is a load-handling device, and a third location is a load-receiver portion of the balance, where each of the at least two locations has at least two load-receiving positions arranged vertically above each other.
This facilitates picking up the load at one of the ocations and setting it down at the other, particularly in case where the load consists of several load portions hat are handled simultaneously.
As mentioned above, the locations where the load is set down are to be selected from no more than three locations whose functions are advantageously coordinated in the operation of the balance. One of these locations can be the stationary holding location for the weighing load, particularly a load consisting of weights (also called mass standards). An apparatus incorporating this feature of the invention is distinguished by having a stationary holding location for the load, particularly a load consisting of weights, with at least two cantilevered portions such as prongs, arms or the like, which are arranged vertically above each other and allow free access from below.
It is advantageous to combine a stationary holding location of this kind, e.g., for weights, with a handling device that has a corresponding vertical arrangement of positions, because this allows the use of a simple configuration without separately movable parts for the handling device. However, it is also possible to configure only the handling device by itself according to the inventive design, for example for performing the process of simultaneously moving a plurality of load portions that are supported in an arrangement of vertically stacked positions. An apparatus incorporating this feature of the invention is distinguished by having a handling device with at least two cantilevered portions such as prongs, arms or the like that are arranged vertically above each other. Finally, in an advantageous embodiment of an apparatus according to the invention, the eccentric loading error is avoided by using a vertically stacked arrangement of the load-receiver devices of a balance, i.e., weighing pans, weighing baskets, or arms for suspending the load, coupled to the force transducer by way of a force-transferring member located either above or below the vertical arrangement of load-receiver devices.
Because the load-receiver devices are vertically stacked either above or below a force-transferring member introducing the load to the force transducer, the centers of gravity of the load portions resting on the load-receiver devices are essentially in vertical alignment, so that the line of action of the combined weight force is spatially fixed in relation to the force-transferring member. Consequently, with the center of gravity not shifting around, the inventive apparatus is free from eccentric loading errors.
It is entirely within the scope of the invention to arrange the force transducer, also referred to as a weighing cell, at the top and use a pendulous configuration for the load-receiver devices. However, in the interest of operating convenience, particularly in a robot-operated balance using the load-handling device described above, as well as for other reasons, the preference goes to a vertical arrangement of the load-receiving devices above the force-transferring member as will be described in the context of the drawing.
The design of load-receiver devices for a balance poses a particular problem in the case where load-handling devices are to be used for putting the weighing objects on the balance. The use 6f robots of this kind keeps the weighing objects and the balance from being touched by an operator. The design task to be solved is particularly challenging in the case where a plurality of load-receiving devices is involved.
According to the invention, the solution to the problem lies in a design for each of the load-receiving locations where all or at least a part of the respective load-supporting devices are configured in the shape of forks with prongs that are open to one side. This basic design is of inventive significance regardless of an overall arrangement of a plurality of devices and whether a plurality or only a single device are provided in any given case. Although load-receiving devices of a perforated, non-continuous kind in the shape of grills or baskets are known per se, the available state-of-the-art devices are unsuitable for use with a robot because the perforations or slits in the grill patterns are enclosed, unlike a fork where the interstices between the prongs are open to one side. The significance of the concept of fork prongs that are open to one side will become evident from the following description of a preferred embodiment. It will also be made clear that an analogous configuration of the load-handling device is of advantage.
Further details of the invention will become clear from the following description of embodiments that are illustrated in the drawings.